What to expect when Imam Mahdi (AJ) appears

The day when by the will of Allah (SWT) Imam Mahdi (AJ) will appear, do not think that some kind of miracle will happen and the world will be reformed in one day. Rather by efforts and sacrifices (of the believers), the oppressors will be defeated and isolated. And like some corrupt simple people, if you think that for preparing for the appearance of that noble one, must try to fill the world with oppression and corruption, then…“verily we are from Allah (SWT) and will return to Allah (SWT)”.
— Sahifeyeh Imam, Vol. 21, Page 448, Imam Khomeini (RA).